Yonhap (Korea state news) :

The National Assembly voted Wednesday to demand President Yoon Suk Yeol lift emergency martial law.

Under the Constitution, martial law must be lifted when a parliamentary majority demands it.

Of the 300 members of parliament, 190 were present and all 190 voted in favor of a motion demanding the lifting of martial law. With the motion's passage, the martial law declaration is void, according to the parliamentary speaker's office. [1]

[1] https://en.yna.co.kr/view/AEN20241204001651315

Article 77 of the Korean Constitution:

"1. When it is required to cope with a military necessity or to maintain the public safety and order by mobilization of the military forces in time of war, armed conict or similar national emergency, the President may proclaim martial law as prescribed by law.

2. Martial law shall be of two types, extraordinary martial law and precautionary martial law.

3. Under extraordinary martial law, special measures may be taken with respect to the necessity for warrants, freedom of speech, the press, assembly and association, or the powers of the Executive and the Judiciary as prescribed by law.

4. When the President has proclaimed martial law, he shall notify the National Assembly without delay.

5. When the National Assembly requests the lifting of martial law with the concurrent vote of a majority of the total members of the National Assembly, the President shall comply."

https://www.constituteproject.org/constitution/Republic_of_K... page 16

Something that is missing in all western news that I have seen so far is why the president did that?

The only info is that he said that it is to protect the democracy from north Korean supporters in the opposition, but no more details.

Does anyone know which action of the opposition triggered that? And why so suddenly? Also, might it be reasonable grounds to his claims or is he just clearly using the martian law against his political opponents?

Something a little bit scary around the world is that it looks like that bad guys are quite active and good at disrupting democracies and elections in free countries (France, us, Romania, moldovia, ...) whereas it looks like that the western world has really low influence to disrupt these countries for a political change.

Can someone ELI5 if this is a huge deal (like declaring martial law would be in the west), or if this is the equivalent of a "government shutdown" when similar budget impasses occur in the US?

The article points out that even some in his own party have denounced the move.

I’m no expert but I find it hard to believe there’s much pro-PRK sympathy in Seoul among a major political party.

Considering this is making western news, it seems the president has over-played his hand. Hopefully this passes quickly.